The present invention relates to a method and apparatus for supervising or controlling a digital transmission system. Transmission systems, particularly submarine transmission systems, include repeaters to amplify the signal along the transmission system and it is clearly necessary that these repeaters are arranged to be supervised remotely. A method that is often used in supervising such repeaters in both analogue and digital systems is "loop-back". In this method, in response to a signal from a terminal station, a loop is established at a repeater, which returns signals, sent on the "send" path, back to the same station on the "receive" path. Thus by looping back at various repeaters the location of faults may be established. Such loop-back may be used not only to establish, for example, cable breaks or serious faults in repeaters, but may preferably also be usable to monitor remotely the quality of the system performance and thereby give early warning of system degradation.
A variety of methods can be used to set up the loop and the present invention in one aspect is concerned with supervising the repeater once a loop-back condition has been established to check functions such as signal level in, signal level out, D.C. levels. ecetera.